Monoethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Monoethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focused on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. For example, US 2011/312050 describes a continuous process for the catalytic generation of polyols from cellulose, in which the cellulose is contacted with hydrogen, water and a catalyst to generate an effluent stream comprising at least one polyol.
CN 102643165 is directed to a catalytic process for reacting sugar in an aqueous solution with hydrogen in the presence of a catalyst in order to generate polyols.
As with many chemical processes, the reaction product streams in these reactions comprise a number of desired materials, diluents, by-products and other undesirable materials. In order to provide a high value process, the desirable product or products must be obtainable from the reaction product stream in high purity with a high percentage recovery of each product and with as low as possible use of energy and complex equipment.
In known processes to make glycols, such as the hydrolysis of ethylene oxide, the glycols are usually present at high dilution in a solvent, typically water. The water is usually removed from the glycols by distillation. Subsequent purification of the glycols is then carried out by fractional distillation.
When glycols are produced by hydrogenolysis of sugars or sugar alcohols, a mixture of glycols is produced. The main glycol constituents in the reaction product stream are monoethylene glycol (MEG), monopropylene glycol (MPG) and 1,2-butanediol (1,2-BDO). The separation of these glycols by fractional distillation is problematic due to the similarity in boiling points, particularly between MEG and 1,2-BDO. The necessary specification for commercial fibre-grade MEG is 99.6 wt % purity. Other grades, such as the lower-value solvent grade MEG (98+wt %) do also exist. It is impossible to recover a high proportion of the MEG at either of these levels of purity from a mixture of MEG and 1,2-BDO by fractional distillation alone due to the formation of MEG/1,2-BDO azeotropes at pressures at which the temperatures required for distillation would not result in the decomposition of glycols. Different methods for the separation on MEG and 1,2-BDO are, therefore, required.
U.S. Pat. No. 4,966,658 is directed to the separation of a mixture of 1,2-BDO and MEG using a process known as azeotropic distillation in which an azeotrope-forming agent is added to the mixture before distillation in order to facilitate separation. A similar process is described in U.S. Pat. No. 5,423,955 for the separation of 1,2-BDO and MPG. Azeotropic distillation can lead to an increase in relative volatility between the components but also leads to further process steps in order to remove the azeotrope forming agents.
The potential application of pressure swing distillation to a number of pressure sensitive binary azeotropes is described in J. P. Knapp and M. F. Doherty, Ind. Eng. Chem. Res., 1992, 31, 346-357. The application of this process to glycols is not described.
WO 2004052808 is directed to a process for the separation of tertiary butyl alcohol from diisobutylene using pressure swing distillation. In said process a mixture of tertiary butyl alcohol and diisobutylene is fed to a first column from which diisobutylene is removed as a bottoms stream. An azeotrope of diisobutylene and tertiary butyl alcohol is also removed from the first column and is then fed to a second column operating at a lower pressure allowing tertiary butyl alcohol to be removed as a bottoms stream.
It would be advantageous to provide an improved method suitable for the separation of monoethylene glycol and 1,2-butanediol from a mixture containing these two components.